“话聊”肿瘤干细胞

marrowstem

干细胞(stem cells)是一类未分化的细胞或原始细胞，多潜能分化及自我更新能力是其两个主要的生物学特性。早在1960/1970年代，许多学者已找到实体肿瘤干细胞存在的实验依据：实体肿瘤细胞存在异质性，只有小部分细胞具有克隆形成能力。      7 v* @6 |/ i$ B5 W
    Hamburger 等发现只有l／l000—1／5000的肺癌、卵巢癌与神经母细胞瘤细胞有能力在体外软琼脂培养基上形成克隆(细胞克隆培养)，这一小部分能形成细胞克隆的肿瘤细胞被认为是“癌干细胞(cancer stem cells)”。2003年，Michael Clarke等通过特异性的细胞表面标志率先在人乳腺癌中分离纯化出乳腺癌干细胞，其只占肿瘤细胞的2％。通过有限稀释分析发现其在NOD／SCID小鼠的成瘤能力至少是其他肿瘤细胞的50倍，即使少于100个细胞也能在小鼠体内成瘤，而且能在体内连续传代。研究还发现由这种乳腺癌干细胞形成的小鼠移植瘤与原先乳腺癌干细胞来源的人乳腺癌组织具有相同的组织类型，即移植瘤既含有这种乳腺癌干细胞又具有其他肿瘤细胞。提示极强的自我复制更新能力与不断分化能力是乳腺癌干细胞两个主要特性。一年后，Peter Dirks等在脑肿瘤中基于CD133标志物通过流式细胞仪分离到一小部分细胞，CD133阳性细胞比阴性细胞具有更强的成瘤性（接种NOD／SCID小鼠）。随后几年，类似的实验在肝癌、卵巢癌、食管癌、大肠癌等实体瘤中得到证实。. H7 M4 b% _4 i6 _4 Z1 o4 `
         正常组织干细胞与肿瘤细胞存在很多共同点，迄今，肿瘤干细胞起源于何种细胞尚未有定论，目前有两种假说：(1)来源于正常干细胞，其获得较少突变即有可能恶性转化，而且正常干细胞存活时间较长，有更多的突变机会形成肿瘤干细胞；(2)一些已经开始分化的原始细胞或成熟细胞也有可能在癌变以前重新获得自我更新能力，经历突变形成肿瘤干细胞。目前高通量的细胞分选系统主要有：磁性细胞分选系统和流式细胞技术(FACS)。分离过程主要分为两步：首先分别按各细胞表面标志进行细胞群的筛选，各细胞群按不同细胞浓度接种NOD／SCID小鼠，观察肿瘤生长，比较各组成瘤能力，筛选出优势细胞群。其次针对几种优势细胞表面标志进行组合(例如A+B-C+)，筛出各组合的细胞群，各按不同细胞浓度接种NOD／SCID小鼠，比较各细胞群成瘤能力，确定目的细胞。3 z. P. j. K  J5 C+ c" a1 }! I
            然而也有一些学者提出不同的观点。2007年，Andreas Strasser等利用老鼠肿瘤细胞接种老鼠实验证实至少大于10%肿瘤细胞能够成瘤。2008年，Sean Morrison等利用NOD/SCID IL2Rγnul老鼠（在NOD/SCID基础上使NK活性失活），能够使近25%～27%的人黑色素肿瘤细胞独立形成新的肿瘤。以上研究有别于其它基于NOD/SCID老鼠评价人肿瘤细胞的实验，因为NOD/SCID老鼠还具有NK活性，因而只有一小部分肿瘤细胞能够成瘤。Hatfiedl等先把老鼠肺癌和乳腺癌细胞系单克隆，然后每个单克隆细胞接种老鼠评价成瘤性，结果绝大部分单细胞克隆具有成瘤性能力。Fengzhi Li等利用类似的方法评价了多种人肿瘤细胞，也得到类似结果。以上结果显示采用不用的方法有不同的结果，但有一点很明确，即肿瘤细胞对环境的适应能力具有明显的异质性，这个特性并不等同于肿瘤干细胞概念。
" C" n! T: ]2 p' S8 W7 l          无论如何，实体瘤干细胞的发现与研究仍具有重要的意义。（1）乳腺癌和脑瘤等肿瘤干细胞的发现、分离及鉴定证明只有小部分肿瘤干细胞才有成瘤能力并维持肿瘤显型。而这一小部分肿瘤干细胞对治疗具有更强的抗性，这解释了肿瘤治疗后的复发转移。(2)肿瘤的发病机制尚未阐明，对实体肿瘤干细胞遗传属性、生物学行为、信号转导等进行系统研究，有助于从根本上阐明肿瘤的发生发展机制。(3)传统肿瘤治疗都是针对所有的肿瘤细胞，而且认为肿瘤细胞具有功能同质性。而实际上如果肿瘤是源于肿瘤干细胞，是一种干细胞疾病，我们先前诸多关于肿瘤发生发展机制、细胞信号途径等的研究成果需要重新评价，因为肿瘤干细胞的生物学行为与其他肿瘤细胞可能存在质的差别。最新研究表明，盐霉素对乳腺癌干细胞有很强的杀伤作用，有效的治疗需要选择性杀伤肿瘤干细胞，这对传统治疗提出极大的挑战。(4)通过对肿瘤干细胞基因表达谱、生物学行为以及功能的不断深入研究，可以阐明其遗传机制、恶性转化路径，从而发掘针对于细胞的治疗新靶点，为临床彻底根治肿瘤展示了新希望。
$ x7 }$ a  y) U8 F7 m8 l9 B0 E7 a# a          总之，实体肿瘤干细胞的发现无疑给全世界学者一个很大的惊喜，而就像对待其他新事物一样我们需要对其进行更多的研究。2 {$ S+ f" W; K" Y' x/ g
（转载 作者郑智国）

marrowstem

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任何研究都不是无根之水，Sean Morrison 在07年就在Nature上发了篇Letter试图推翻肿瘤干细胞理论，更为神奇的是，据06年4月5日的一则新闻显示，Sean Morrison在那个时候还深信着这一理论并在努力寻找肿瘤干细胞表面Marker进而作为癌症治疗的靶点。$ z5 F" G8 S' z. |  m
      下面是06年前Sean Morrison对肿瘤干细胞理论的狂热推崇(06年4月5号发在HMMI上的news，题目是Researchers Target Cancer Stem Cells' Unique Vulnerability ）：* e0 R/ Q( R1 H3 S7 t+ |2 u
         New research into the unique properties of stem cells indicates there is a useful difference between cells that keep the blood system healthy and the stem cells that make leukemia lethal, scientists report. 7 C: @. x' s0 x; a
          Discovery of the difference is important, because experiments in mice now show that the stem cells promoting leukemia can be killed by rapamycin, a drug that has already received Food and Drug Administration (FDA) approval, said Sean J. Morrison, a Howard Hughes Medical Institute investigator at the University of Michigan Life Sciences Institute who led the research. ( g" O+ I  z  a6 }7 v" {
         “This is proof-of-principle that it’s possible to identify differences between cancer stem cells and normal stem cells, and then therapeutically exploit these differences to eliminate cancer stem cells without harming the normal stem cells.”
+ O# T/ T9 g/ \! k       Sean J. Morrison  1 S; x+ k% o& q+ O; @' Q7 k+ _
      “Our study demonstrates that it's possible to identify mechanistic differences between normal stem cells and cancer stem cells, and that the differences can be exploited therapeutically,” Morrison explained. A full report on the work is published in the April 6, 2006, issue of the journal Nature. Omer H. Yilmaz, a graduate student in Morrison's lab, is the lead author of the report. Colleagues Riccardo Valdez, Brian K. Theisen, David O. Ferguson, are co-authors, as well as collaborators Wei Guo and Hong Wu from the University of California - Los Angeles. $ M" ~% R# b0 i1 f* |4 Y5 b3 G
          Worldwide there is a huge amount of research underway on stem cell biology, and researchers are hoping to learn how stem cells can be manipulated to treat diseases, including cancer, or used to replace failing organs. The new work begins to answer how those stem cells involved in cancer resemble and differ from the normal stem cells that keep bone marrow and other tissues healthy. $ T$ t- u" {( E7 L1 p% i
          Finding exploitable differences between cancer cells and normal cells has ranked among the most important goals of cancer research. According to Morrison, the idea is to find some vulnerability, some chink in the cancer cell's armor, that will enable scientists to kill cancer cells while sparing cells needed for normal functioning, such as bone marrow cells. This would allow the development of anti-cancer therapies that are more effective and that have less toxic side-effects to normal tissues.
: f5 l, N" r  ?* }! }        According to Morrison, “Cancer stem cells -- cells thought to initiate and maintain tumors -- share many properties with normal stem cells, making it difficult to design cancer treatments that target cancer stem cells without killing normal stem cells. The problem is that damaging normal blood-forming stem cells during treatment for leukemia would severely compromise the immune system,” rendering patients extra-vulnerable to infections, he said. . K; l* e* V! ?. V
         But now, evidence from mouse studies in Morrison's lab shows that rapamycin can reduce the number of existing leukemic stem cells, and prevent the growth of those that remain, while restoring the function of normal stem cells needed to maintain a healthy supply of blood. The treatment also restores the animals' ability to replenish the supply of immune system cells after the bone marrow has been depleted. 1 l! [3 n2 e3 g4 p. h4 ]7 Y' ?) t) m
            The new work is founded on the observation that there are inherent distinctions among different types of stem cells. In leukemia, Morrison said, “it was discovered that not all cancer cells have an equal ability to proliferate. Some cancer cells have only limited ability to grow, while a smaller subset of cells has tremendous ability to proliferate, to transfer disease in transplantation, and kill. This malignant subset is called cancer stem cells because they are similar in function and appearance to normal stem cells.”
( d8 `2 L; x, {          So the problem, Morrison said, is to find some exploitable difference between the two. To cure leukemia “it is necessary and sufficient to kill the cancer stem cells, but if they are so similar to normal stem cells, how do we kill them without also killing normal stem cells in the same tissue?” Morrison asked.
  J& _! J8 `( Z- Y  mThe answer is to seek small, rare differences in the mechanisms that maintain the leukemic stem cells compared to normal blood-forming stem cells. And cancer researchers have found one such difference: a gene called Pten, which has opposite effects on the two types of stem cells: When scientists remove or inactivate the Pten gene, the supply of normal stem cells declines, while the growth of cancer stem cells increases dramatically.
; [  B# J3 [' f9 J+ v- ^That offered a golden opportunity.
. ~  z8 v# k3 U4 a; m8 @“We were able to target this Pten pathway with the drug rapamycin, which killed leukemic stem cells without harming normal stem cells,” Morrison said. “So this is proof-of-principle that it's possible to identify differences between cancer stem cells and normal stem cells, and then therapeutically exploit these differences to eliminate cancer stem cells without harming the normal stem cells.”
3 Z7 m: T" D+ P5 d) v+ q& B           Although it is still early in the research process, and experiments have not yet moved beyond those conducted in mice, it seems likely that there will soon be progress in the clinic, since rapamycin is already in use as an FDA-approved anti-cancer drug.
, A7 u  i" s: h: f  p# v           “I believe these results will have an impact on current clinical trials which are being performed with rapamycin,” Morrison added. “They offer hope that it will be possible to develop more effective and less toxic chemotherapy drugs based on a better understanding of stem cell self-renewal.” ! k. K6 h, h# Y( Z
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         从肿瘤干细胞理论的信徒到掘墓者，做出这一转变，Sean Morrison肯定经历很多旁人不知的苦楚，那些06年前在Sean Morrison手下苦苦寻找肿瘤干细胞marker的前辈估计是成了烈士了，你们辛苦了～
7 @) R0 ~) ]  i' K. i9 d3 N3 ~       （摘编自杨轩博文）

marrowstem

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& ~7 g6 y" H8 L1 Y, O肿瘤干细胞的理论一直很诱人，按我的理解就是说，肿瘤里大部分细胞其实都是“好”的，只有极少数的真正“坏”的肿瘤干细胞，他们能无限增值扩散之后能产生新的肿瘤，他们正是肿瘤越长越大的动力源泉。这个理论很诱人，因为一旦找到能够把这些真“坏”的细胞分辨出来的marker，那就可以针对性设计药物，进而显著控制癌症甚至治愈癌症。无数青年才俊正在这一诱人前景的吸引下投身于寻找“肿瘤干细胞”，干掉“肿瘤干细胞”的梦想中，现在，他们的努力可能会付之东流。) C% Q5 _' X, _- H
        HHMI的Sean Morrison（据说是干细胞研究的少掌门……）08年12月4日的一篇Nature 跳出来说，至少在melanoma里面，有25%～27%的细胞都是能独立形成新的肿瘤，进而都可以被认为的传统意义上的“坏”细胞。而且四分之一的“坏”细胞和另外的好细胞表面的50种marker都不存在显著差别。, T$ V0 z7 C/ E* c" T
        传统的肿瘤干细胞理论主要是基于NOD/SCID mice的实验，就是把一定数量的肿瘤细胞打到这些免疫缺陷的老鼠里，看看长出了多少个独立的肿瘤，进而统计出有多少细胞能够致瘤，结论是能够致瘤的癌细胞数量占总细胞数的极少部分，进而支持了肿瘤干细胞理论。但NOD/SCID mice模型有个严重的缺点，就是残留的NK细胞活性，现在Sean Morrison整了新的“more severely immune-compromised mice, called NOD/SCID IL2Rγnull. These mice are missing the gene for a receptor protein required for immune cells known as natural killer cells to function properly. The team found that 250,000 times as many melanoma cells formed tumors in this modified assay as compared to the standard NOD/SCID mice. ”
7 p+ V) [2 G" |( T      现在好了，没有NK细胞活性的新老鼠模型里，“肿瘤干细胞”一下子多了250，000倍！换句话说，就是打100个melonoma 细胞到这种老鼠模型里，有25个细胞能形成肿瘤；而如果打单一melanoma细胞到老鼠里，那么会有27%的老鼠产生肿瘤（这是个新方法）。
3 X# A! q' g+ ^+ U; D0 X# L/ K          House最新一季的第一集正是拿cancer stem cell来忽悠，估计以后再想拿这个说事儿就得三思了。3 X5 A& U; p8 {- D' ~) g
        还有，NK细胞可能真的是癌症免疫治疗的关键先生呢！
* y. ]* R; s  G5 @3 q0 y1 j   （摘编自杨轩博文）

marrowstem

Melanoma Spawns Tumors with Deadly Efficiency
, J+ X0 p0 \7 `" ^' E        By examining the tumor-promoting properties of individual cancer cells, Howard Hughes Medical Institute scientists have found that melanoma cells spawn new tumors with deadly efficiency. The new studies show that at least one of every four melanoma cells has the capacity to seed the development of new tumors. & a( _% H* x! g
          HHMI investigator Sean Morrison and his colleagues at the University of Michigan Medical School report their findings in the December 4, 2008, issue of the journal Nature. Elsa Quintana and Mark Shackleton, postdoctoral fellows in the Morrison laboratory, performed the experiments, and Timothy Johnson, director of the University of Michigan's Multidisciplinary Melanoma Program, provided melanoma samples with patients' consent.
8 p$ E" Q( j+ L# X. M  B! a) d         “We think the underestimation of tumor-causing cells is a general problem in many cancers, not just specific to melanoma.”7 b5 ~- l7 r+ w! \0 r
Sean J. Morrison  
% i! u( v% a) K- ~( r         For years, cancer researchers have debated whether most cancer cells are equally likely to cause new tumors. One prevailing hypothesis among cancer researchers is that only cancer stem cells -- a tiny subset of highly prolific cells -- can actually give rise to tumors. As recently as January 2008, a prominent report in Nature estimated that as few as one in a million melanoma cells is capable of prompting tumor growth. + g: c" e/ Z8 N
           Indeed, researchers have put much effort into devising new ways to identify cancer stem cells by virtue of distinctive molecular markers that may separate them from other cancer cells. Once identified, cancer stem cells may be targeted by new cancer drugs that prevent tumor formation by hitting the source of the cancer. Some have even suggested that cancer might be more effectively treated using therapies that are designed to target small subpopulations of cancer stem cells, rather than trying to eliminate all cancer cells.
. e2 y8 r: K" x( B0 o0 d  A            A leader in stem cell research, Morrison said he has an open mind about the potential existence of cancer stem cells. But he also believes that additional work is required to detect tumorigenic human cancer cells and to assess the frequency of those cells in different types of cancer. “Identification of the full spectrum of cells with the potential to drive the growth and progression of human cancers will be required to develop more effective new treatments,” he said.
: v2 h6 V. C! S           Scientists normally begin experiments to assess how efficient cancer cells are at forming tumors by first injecting large numbers of cancer cells into mice with weakened immune systems. After that has been done, the researchers count how many tumors grow in the mice. They can then compare the number of tumors formed with the number of cells injected and arrive at a rough estimate of what percentage of cancer cells were able to form new tumors. & q2 ?) h8 f7 Z
According to Morrison, researchers knew that the mice typically used for these experiments, known as NOD/SCID mice, were not perfect models. Even though they have weakened immune systems, the mice retain some defensive immune cells that can suppress the growth of human cells.
. N* o  L& a. ]( Z( t- C. @3 N          But, said Morrison, "it was thought that the assays only moderately under-estimated the frequency of cancer cells with the potential to form tumors. People thought improving the mice might improve sensitivity tenfold or a hundredfold, but they thought 'why bother doing the work to go from one in a million to one in ten thousand, since that would not change the conclusion that only rare cancer cells have the potential to proliferate extensively?'" explained Morrison. ! u7 }; N# S3 l, l4 t' A
           Morrison thought it was important to continue to challenge the existing model, so his team identified multiple improvements in the assay, including transplanting human melanoma cells into more severely immune-compromised mice, called NOD/SCID IL2Rγnull. These mice are missing the gene for a receptor protein required for immune cells known as natural killer cells to function properly. The team found that 250,000 times as many melanoma cells formed tumors in this modified assay as compared to the standard NOD/SCID mice. % G: ?4 b9 K5 k5 g1 o5 w' E% j
The team tested the melanoma cells by transplanting batches of cells and - more importantly - individual melanoma cells. For batches of melanoma cells, they found that about 25 percent of the cells formed tumors. About 27 percent of the single melanoma cells formed tumors when transplanted into the NOD/SCID IL2Rγnull mice.
% Z$ C/ _2 [/ j' e& B" l          "As far as we know, this is the first time anyone has been able to show that individual cells from human cancers can efficiently form tumors. The result provides direct support to the idea that many cancer cells are capable of proliferating extensively and forming tumors," noted Morrison. The finding suggests that targeting just a small subset of melanoma cells will not be enough to stop the cancer from growing. : f) K' n$ X& m. d9 x4 G
           "We think the underestimation of tumor-causing cells is a general problem in many cancers, not just specific to melanoma," says Morrison. "Researchers in the field need to go back to optimize the assays they have been using and to re-evaluate the evidence underlying cancer stem cell models."
' K$ p- n% W) D& _          An important premise of the cancer stem cell model is that the cancer stem cells that cause tumors are distinguishable at the molecular level from other cancer cells. This individuality is an important property that scientists would theoretically be able to exploit in designing new therapies to target only tumor-causing cells. But when Morrison's team looked at 50 different molecular markers found on the surface of the melanoma cells, it could find no differences between the tumor-causing cells and those that did not spawn tumors. : c6 s6 B* i( p6 ?# V* `0 c/ T
          "The simplest interpretation is that most melanoma cells have a 25 percent chance of forming a tumor…and that melanoma does not contain intrinsically distinct types of tumorigenic and non-tumorigenic cancer cells," said Morrison, “but it is impossible to prove that because somebody could come along and discover that the fifty-first marker does distinguish cancer stem cells from normal cancer cells.”
' R- O& c, m9 E0 m          Morrison acknowledges that it is possible that cancer stem cells are simply more common in certain cancers. But, he said, “as is often the case, the truth is probably somewhere in between the two prevailing theories. ' U: o- [7 I; R" B4 n  M- z
“We expect that some cancers will follow a cancer stem cell model, while other cancers will not,” he said. “Many cancers will be like melanoma, a good old-fashioned cancer in which every cell is bad.”
3 X* `* R) }) e, H' ~, F          The approaches used in the current study should help researchers analyze other cancers to more accurately estimate the frequency of cells with tumorigenic potential, as well as to study how individual cancer cells change over time. "The ability to study tumorigenesis from single human cancer cells opens new doors,” Morrison said. "This does make things a lot more complicated, but cancer is a resourceful disease. To defeat it we must do the work to understand its complexity."

marrowstem

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6 K2 g+ U3 L) V% G" X9 nNature上的Sean Morrision质疑肿瘤干细胞的paper：: R5 B  F+ x7 Y$ f* b! L

marrowstem

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1 H: V/ x  i. H1 kCell上的Review：) @  x. R' I: M/ y9 R- M$ V7 {

tomyang1987

很有帮助

xiongliangxl

膜拜~学习中~孰是孰非，等后人评判~

chumingliang

我还是相信肿瘤干细胞的理论，因为我希望这个突破是治愈肿瘤的希望

toni8038

如果把肿瘤组织看成是一个社会组织的话，那它肯定要有组织者，即头头，因此个人觉得应该有肿瘤干细胞的存在。